Tuning the position of the redox couples in materials with NASICON structure by anionic substitution

Previous studies on insertion of lithium into rhombohedral forms of Fe-2(SO4)(3) and Li3Fe2(PO4)(3) have determined the position of the Fe3+/Fe2+ redox couple to be at 3.6 and 2.8 eV, respectively, below the Fermi level of lithium. Lithium insertion into rhombohedral LiFe2(SO4)(2)(PO4) with NASICON framework exposes the Fe3+/Fe2+ redox couple at 3.3-3.4 eV below the Fermi energy of lithium. Unlike Fe-2(SO4)(3), the insertion of lithium into the structure of LiFe2(SO4)(2)(PO4) is accomplished as a single-phase solid solution, which makes it attractive as an electrode material for rechargeable lithium batteries.